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Class 11
>>Applied Mathematics
>>Straight lines
>>Introduction
>>In the coordinate plane, the points F( -
Question
In the coordinate plane, the points F(−2,1),G(1,4) and H(4,1) lie on a circle with center P. What are the coordinates of point P ?
A
(0,0)
B
(1,1)
C
(1,2)
D
(1,−2)
Medium
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Solution
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Correct option is B)
Let H≡(x
1
,y
1
)=(4,1)
G≡(x
2
,y
2
)=(1,4)
F≡(x
3
,y
3
)=(−2,1)
P≡(x,y)
slope of m
r
=
x
2
−x
1
y
2
−y
1
=
1−4
4−1
=−1
slope of m
t
=
x
3
−x
2
y
3
−y
2
=
−2−1
1−4
=1
Formula for x-coordinate of the center of the circle is given as:
x=
2(m
r
−m
t
)
m
r
m
t
(y
3
−y
1
)+m
r
(x
2
+x
3
)−m
t
(x
1
+x
2
)
∴x=
2(−1−1)
(−1)(1)(1−1)+−1(1+(−2))−1(4+1)
∴x=
−4
0+1−5
=1
y=−
m
r
1
[x−
2
x
1
+x
2
]+
2
y
1
+y
2
∴y=−
−1
1
[1−
2
4+1
]+
2
1+4
∴y=[
2
2−5
]+
2
5
=1
∴P≡(1,1)